The goal of the proposed project is to provide experimental data on the fine structural organization of subcortical afferents in the individual subdivisions of the motor thalamus in non-human primates. The project is specifically focused on the identification and characterization of basal ganglia (pallidal and nigral) and cerebellar (from individual deep cerebellar nuclei) afferents to the motor thalamus and distribution of their synaptic terminals on the two types of cells: thalamocortical projection neurons and local circuit neurons. These data are needed for understanding the organization of the neuronal circuity involved in the motor control at the thalamic level. The project is justified by the fact that such information is currently lacking in regard to primates and generalizations, based on the data obtained in other species, are invalid: the light microscopic data indicate that the topographical relationships of subcortical afferents in the primate motor thalamus essentially differ from that in lower mammals and a few available EM observations suggest that the synaptic organization in the primate motor thalamus is more complex than that in nonprimate species. We propose: (1) to study the features and the termination mode of axon terminals of pallidal, nigral, and cerebellar efferents in individual cytoarchitectural subdivisions of the motor thalamus after their identification by EM autoradiography; (2) to analyze quantitatively the density and distribution of specific afferent terminals on different type thalamic neurons in individual motor nuclei; (3) to determine at the light and EM levels qualitative and quantitative features of thalamocortical projection neurons identified by retrograde HRP labeling from the cortex; (4) to gain understanding on light and EM features of local circuit neurons and their relationships with projection neurons in individual motor thalamic nuclei using HRP and GAD immunocytochemistry experiments; and (5) to compare qualitative and quantitative parameters of synaptic organization in individual subdivisions of the primate motor thalamus. The results of these studies will provide information on the organization of motor control at the thalamic level in primates which is crucial for understanding the mechanisms of dyskinetic disorders in humans.